The illumination of aircraft instruments, i.e., indicator and switch displays, presents unique problems not found in other lighting applications. The safe operation of an aircraft requires that instrument displays be illuminated so that crew members may read the displays during both daytime and nighttime operation of the aircraft. During daytime operation, the displays must be brightly illuminated to overcome the effects of direct or diffuse sunlight impinging on a display and washing out the display. During nighttime operation, however, the displays do not need to be as brightly illuminated. In fact, for certain types of aircraft, such as military aircraft in which crew members wear night vision goggles (NVG), it is imperative that the illumination of the displays be reduced during nighttime operation of the aircraft. Night vision goggles amplify light in a limited wavelength band. Because light is amplified, the night vision capability of crew members wearing night vision goggles is improved. The amplification provided by night vision goggles requires that the lighting level of instrument displays be reduced from the bright daytime lighting level to a lower nighttime lighting level so that the night vision goggles are not overpowered by the brightness of the display. If the illuminated displays are not dimmed from the daylight lighting level, the night vision goggles may become temporarily nonfunctional, effectively blinding the crew member(s) wearing the goggles and placing both the crew and the aircraft in peril.
One approach used by the prior art to provide both daytime and nighttime illumination of an aircraft instrument displays, is to apply a variable voltage to the lamps that light the display. Conventional instruments typically include one set of lamps that light a single display. The application of a low voltage causes the set of lamps to light the display to a dim, i.e., nighttime, illumination level. The application of a high voltage causes the lamps to light the display to a bright, i.e., daytime, illumination level.
In this prior art approach, if the display is to be readable by crew members wearing night vision goggles, a night vision goggle compatible filter is inserted between the lamps and the display. The night vision goggle compatible filter further reduces the amount of light emanating from the instrument display in the wavelength band that the night vision goggles are sensitive to (e.g., in the 600-900 nm range). One problem associated with this approach is that the night vision goggle compatible filter remains in use during the daytime illumination of the display and, unfortunately, reduces the amount of light that is detectable by the human eye. Furthermore, the special night vision filter does not duplicate the visual colors normally specified for aviation displays, i.e., amber and red. As a result, during daytime operation of the aircraft, the daytime lighting level of the instrument display is reduced from its maximum brightness level and, thereby, increases the likelihood that the display will be washed out by direct sunlight. Also, during daytime operation the visual color appearance must be comprised to achieve nighttime compatibility with the goggles.
Other problems associated with this prior art approach include the size and inefficiency of the control circuits that apply the variable voltage to the lamps. For example, in one type of prior art control circuit, operational amplifiers are connected as buffers to control the voltage applied to the instrument lamps. Unfortunately, such a circuit is relatively inefficient because the voltage drop through an operational amplifier configured as a buffer is relatively high. Equally important, the voltage drop through the buffer reduces the magnitude of the voltage available to the instrument lamps and, thus, reduces the brightness of the lamps during daytime operation. In addition, because prior art control circuits typically use discrete electrical components, they hinder efforts to reduce the size and weight of the parts used in aircraft.
Another prior art approach to providing dual illumination levels uses two sets of instrument displays. One set of instrument displays is used during the nighttime operation of the aircraft and one set of instrument displays is used during the daytime operation. Each of the nighttime instrument displays is used during the daytime operation. Each of the nighttime instrument displays has a night vision goggle compatible filter so as to make it readable by crew members wearing the goggles. While not necessary in all cases, each of the daytime instrument displays may have a filter designed to improve the readability of the instruments during the daytime operation of the aircraft. While this approach overcomes the common filter problem discussed above, a significant problem arises with this approach, namely, the additional weight and space requirements of the second set of instrument displays.
As can be appreciated from the foregoing discussion, there is a need for an aircraft instrument display that can be lighted to satisfactory nighttime and daytime illumination levels. The instrument display should have a night vision compatible filter that is used for nighttime illumination of the display but not for daytime illumination. The control circuit used to control the instrument lamps should be efficient and compact. The present invention is directed to a novel dimming circuit and a dual display instrument designed to achieve these results.